A pneumatic tire having a radial carcass reinforcement comprises, in a known manner, a tread, two inextensible beads, two flexible sidewalls joining the beads to the tread and a rigid crown reinforcement or “belt” placed circumferentially between the carcass reinforcement and the tread.
The tire belt generally consists of superposed rubber plies which may contain metal or textile reinforcing threads, generally arranged parallel to one another within a given ply.
In particular, this belt may comprise one or more crown plies known as “protective” plies, in general located beneath the tread, the role of which protective plies is to protect the rest of the belt from external attack, tearing or other perforations. This is for example the general case in the belts of tires for heavy vehicles or civil engineering vehicles.
These protective plies must be sufficiently flexible and deformable so as, on the one hand, to follow as closely as possible the shape of the obstacle on which the belt bears during rolling and, on the other hand, to prevent the penetration of foreign bodies radially towards the inside of said belt. To meet such criteria requires the use, in these protective layers, of reinforcing threads in the form of cords having a high elasticity and a high fracture energy.
Steel cords known as “strand cords”, also described as high elongation cords (HE cords) are normally used, these being assembled by the known technique of stranding and consisting of a plurality of metal strands twisted together in a helix, each strand comprising several steel wires which are also wound together in a helix.
Such elastic strand cords have been described in a large number of patents or patent applications, in particular to reinforce protective crown plies of tires for industrial vehicles such as heavy or civil engineering vehicles (see for example U.S. Pat. No. 5,843,583, U.S. Pat. No. 6,475,636, WO 2004/003287 (or US 2005/0183808), WO 2004/033789 or U.S. Pat. No. 7,089,726, WO 2005/014925 or US 2006/0179813).
However, these protective crown plies reinforced with metal strand cords have a certain number of drawbacks.
Firstly, these strand cords are relatively expensive, this being so on two counts: firstly, they are prepared in two steps, namely by the prior manufacture of the strands followed by assembly by twisting these strands, and, secondly, they generally require their wires to have a high twist (i.e. a very short helix pitch), this twist being essential in order to give them the desired elasticity but leading to low manufacturing rates. This drawback of course has repercussions on the cost of the tires themselves.
Other known drawbacks of these metal cords are their sensitivity to corrosion, their weight and their relatively large size (outside diameter).